Charge Tolerant Microchannel Heat Exchanger

ABSTRACT

A microchannel heat exchanger has an upper portion connected in fluid communication with an undivided header, and a lower portion connected in fluid communication with the undivided header at a location that is vertically lower on the undivided header than the upper portion. Under normal steady state operating conditions, the heat exchanger may condense a first amount of liquid phase refrigerant, wherein the first volume of liquid phase refrigerant substantially fills the lower portion, and wherein substantially none of the refrigerant is condensed into liquid phase refrigerant in the lower portion, and under abnormal steady state operating conditions, the heat exchanger may condense a second amount of liquid phase refrigerant in the upper portion, wherein an additional amount of the second amount of liquid phase refrigerant in excess of the first amount of liquid phase refrigerant is accommodated in the undivided header as opposed to further filling the lower portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. 119(e) to U.S.Provisional Patent Application No. 61/924,619 filed on Jan. 7, 2014 byStephen Stewart Hancock, entitled “Charge Tolerant Microchannel HeatExchanger,” the disclosure of which is hereby incorporated by referencein its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Heating, ventilation, and/or air conditioning (HVAC) systems maygenerally be used in residential and/or commercial structures to provideheating and/or cooling to climate-controlled areas within thesestructures. Some HVAC systems may comprise a microchannel heatexchanger. However, because a microchannel heat exchanger may comprise atwo-phase refrigerant volume that may be less than 1% of the volume of aconventional heat exchanger, microchannel heat exchangers remainsensitive to liquid refrigerant volume, which can change due toinaccuracies in charging the unit with refrigerant and/or during changesin operating conditions that may cause the refrigerant to change phasesand/or any liquid refrigerant to change density. In some cases, liquidrefrigerant may displace two-phase refrigerant within the microchannelheat exchanger and thereby significantly degrading the performance ofthe microchannel heat exchanger as compared to the performance of themicrochannel heat exchanger when two-phase refrigerant occupies thespace.

SUMMARY

In some embodiments of the disclosure, a microchannel heat exchanger isdisclosed as comprising a first portion connected in fluid communicationwith an undivided header, and a second portion connected in fluidcommunication with the undivided header at a location that is verticallylower on the undivided header than the first portion, wherein ratio ofthe first portion to the second portion is greater than 2:1.

In other embodiments of the disclosure, a method of operating amicrochannel heat exchanger is disclosed as comprising: providing amicrochannel heat exchanger in an HVAC system, the microchannel heatexchanger comprising an upper portion connected in fluid communicationwith an undivided header, and a lower portion connected in fluidcommunication with the undivided header at a location that is verticallylower on the undivided header than the upper portion, wherein the ratioof the upper portion to the lower portion is greater than 2:1;introducing a refrigerant into the upper portion; under normal steadystate operating conditions, condensing a first amount of liquid phaserefrigerant in the upper portion, wherein the first volume of liquidphase refrigerant substantially fills the lower portion, and whereinsubstantially none of the refrigerant is condensed into liquid phaserefrigerant in the lower portion; and under abnormal steady stateoperating conditions, condensing a second amount of liquid phaserefrigerant in the upper portion, wherein an additional amount of thesecond amount of liquid phase refrigerant in excess of the first amountof liquid phase refrigerant is received in the undivided header.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and theadvantages thereof, reference is now made to the following briefdescription, taken in connection with the accompanying drawings anddetailed description:

FIG. 1 is a simplified schematic diagram of an HVAC system according toan embodiment of the disclosure;

FIG. 2 is an orthogonal front view of an outdoor heat exchangeraccording to an embodiment of the disclosure;

FIG. 3 is a partial cutaway oblique view of a plurality of microchanneltubes of the outdoor heat exchanger according to an embodiment of thedisclosure;

FIG. 4A is a schematic view of a conventional heat exchangerillustrating the behavior of liquid refrigerant in the conventional heatexchanger under normal steady state operating conditions;

FIG. 4B is a schematic view of a conventional heat exchangerillustrating the behavior of liquid refrigerant in the conventional heatexchanger under abnormal steady state operating conditions;

FIG. 5A is a schematic view of an outdoor heat exchanger illustratingthe behavior of liquid refrigerant in the outdoor heat exchanger undernormal steady state operating conditions according to an embodiment ofthe disclosure;

FIG. 5B is a schematic view of an outdoor heat exchanger illustratingthe behavior of liquid refrigerant in the outdoor heat exchanger underabnormal steady state operating conditions according to an embodiment ofthe disclosure; and

FIG. 6 is a flowchart of a method of operating a microchannel heatexchanger in an HVAC system according to an embodiment of thedisclosure.

DETAILED DESCRIPTION

In some cases, it may be desirable to provide a charge tolerantmicrochannel heat exchanger for an HVAC system. For example, whereabnormal steady state operating conditions and/or overcharging of theHVAC system may cause liquid refrigerant to occupy a portion of a heatexchanger optimized for transferring heat with gaseous or mixed phaserefrigerant, it may be desirable to provide a charge tolerantmicrochannel heat exchanger for an HVAC system that may provide anincrease in efficiency when operating in an overcharged state and/orunder normal or abnormal steady state operating conditions. In someembodiments, systems and methods are disclosed that comprise providing acharge tolerant microchannel heat exchanger that comprises an undividedheader that is configured to receive excess liquid refrigerant tomaintain and/or increase the efficiency of the charge tolerantmicrochannel heat exchanger. In some embodiments, the charge tolerantmicrochannel heat exchanger may be used in an HVAC system, including,but not limited to, a heat pump system.

Referring now to FIG. 1, a simplified schematic diagram of an HVACsystem 100 is shown according to an embodiment of the disclosure. HVACsystem 100 generally comprises an indoor unit 102, an outdoor unit 104,and a system controller 106. The system controller 106 may generallycontrol operation of the indoor unit 102 and/or the outdoor unit 104. Asshown, the HVAC system 100 is a so-called heat pump system that may beselectively operated to implement one or more substantially closedthermodynamic refrigeration cycles to provide a cooling functionalityand/or a heating functionality.

Indoor unit 102 generally comprises an indoor heat exchanger 108, anindoor fan 110, and an indoor metering device 112. Indoor heat exchanger108 is a plate fin heat exchanger configured to allow heat exchangebetween refrigerant carried within internal tubing of the indoor heatexchanger 108 and fluids that contact the indoor heat exchanger 108 butthat are kept segregated from the refrigerant. In other embodiments,indoor heat exchanger 108 may comprise a spine fin heat exchanger, amicrochannel heat exchanger, or any other suitable type of heatexchanger.

The indoor fan 110 is a centrifugal blower comprising a blower housing,a blower impeller at least partially disposed within the blower housing,and a blower motor configured to selectively rotate the blower impeller.In other embodiments, the indoor fan 110 may comprise a mixed-flow fanand/or any other suitable type of fan. The indoor fan 110 is configuredas a modulating and/or variable speed fan capable of being operated atmany speeds over one or more ranges of speeds. In other embodiments, theindoor fan 110 may be configured as a multiple speed fan capable ofbeing operated at a plurality of operating speeds by selectivelyelectrically powering different ones of multiple electromagneticwindings of a motor of the indoor fan 110. In yet other embodiments, theindoor fan 110 may be a single speed fan.

The indoor metering device 112 is an electronically controlled motordriven electronic expansion valve (EEV). In alternative embodiments, theindoor metering device 112 may comprise a thermostatic expansion valve,a capillary tube assembly, and/or any other suitable metering device.The indoor metering device 112 may comprise and/or be associated with arefrigerant check valve and/or refrigerant bypass for use when adirection of refrigerant flow through the indoor metering device 112 issuch that the indoor metering device 112 is not intended to meter orotherwise substantially restrict flow of the refrigerant through theindoor metering device 112.

Outdoor unit 104 generally comprises an outdoor heat exchanger 114, acompressor 116, an outdoor fan 118, an outdoor metering device 120, anda reversing valve 122. Outdoor heat exchanger 114 is a microchannel heatexchanger configured to allow heat exchange between refrigerant carriedwithin internal passages of the outdoor heat exchanger 114 and fluidsthat contact the outdoor heat exchanger 114 but that are kept segregatedfrom the refrigerant. In other embodiments, outdoor heat exchanger 114may comprise a plate fin heat exchanger, a spine fin heat exchanger, orany other suitable type of heat exchanger.

The compressor 116 is a multiple speed scroll type compressor configuredto selectively pump refrigerant at a plurality of mass flow rates. Inalternative embodiments, the compressor 116 may comprise a modulatingcompressor capable of operation over one or more speed ranges, areciprocating type compressor, a single speed compressor, and/or anyother suitable refrigerant compressor and/or refrigerant pump.

The outdoor fan 118 is an axial fan comprising a fan blade assembly andfan motor configured to selectively rotate the fan blade assembly. Inother embodiments, the outdoor fan 118 may comprise a mixed-flow fan, acentrifugal blower, and/or any other suitable type of fan and/or blower.The outdoor fan 118 is configured as a modulating and/or variable speedfan capable of being operated at many speeds over one or more ranges ofspeeds. In other embodiments, the outdoor fan 118 may be configured as amultiple speed fan capable of being operated at a plurality of operatingspeeds by selectively electrically powering different ones of multipleelectromagnetic windings of a motor of the outdoor fan 118. In yet otherembodiments, the outdoor fan 118 may be a single speed fan.

The outdoor metering device 120 is a thermostatic expansion valve. Inalternative embodiments, the outdoor metering device 120 may comprise anelectronically controlled motor driven EEV similar to indoor meteringdevice 112, a capillary tube assembly, and/or any other suitablemetering device. The outdoor metering device 120 may comprise and/or beassociated with a refrigerant check valve and/or refrigerant bypass foruse when a direction of refrigerant flow through the outdoor meteringdevice 120 is such that the outdoor metering device 120 is not intendedto meter or otherwise substantially restrict flow of the refrigerantthrough the outdoor metering device 120.

The reversing valve 122 is a so-called four-way reversing valve. Thereversing valve 122 may be selectively controlled to alter a flow pathof refrigerant in the HVAC system 100 as described in greater detailbelow. The reversing valve 122 may comprise an electrical solenoid orother device configured to selectively move a component of the reversingvalve 122 between operational positions.

The system controller 106 may generally comprise a touchscreen interfacefor displaying information and for receiving user inputs. The systemcontroller 106 may display information related to the operation of theHVAC system 100 and may receive user inputs related to operation of theHVAC system 100. However, the system controller 106 may further beoperable to display information and receive user inputs tangentiallyand/or unrelated to operation of the HVAC system 100. In someembodiments, the system controller 106 may not comprise a display andmay derive all information from inputs from remote sensors and remoteconfiguration tools. In some embodiments, the system controller 106 maycomprise a temperature sensor and may further be configured to controlheating and/or cooling of zones associated with the HVAC system 100. Insome embodiments, the system controller 106 may be configured as athermostat for controlling supply of conditioned air to zones associatedwith the HVAC system 100.

In some embodiments, the system controller 106 may also selectivelycommunicate with an indoor controller 124 of the indoor unit 102, withan outdoor controller 126 of the outdoor unit 104, and/or with othercomponents of the HVAC system 100. In some embodiments, the systemcontroller 106 may be configured for selective bidirectionalcommunication over a communication bus 128. In some embodiments,portions of the communication bus 128 may comprise a three-wireconnection suitable for communicating messages between the systemcontroller 106 and one or more of the HVAC system 100 componentsconfigured for interfacing with the communication bus 128. Stillfurther, the system controller 106 may be configured to selectivelycommunicate with HVAC system 100 components and/or any other device 130via a communication network 132. In some embodiments, the communicationnetwork 132 may comprise a telephone network, and the other device 130may comprise a telephone. In some embodiments, the communication network132 may comprise the Internet, and the other device 130 may comprise asmartphone and/or other Internet-enabled mobile telecommunicationdevice. In other embodiments, the communication network 132 may alsocomprise a remote server.

The indoor controller 124 may be carried by the indoor unit 102 and maybe configured to receive information inputs, transmit informationoutputs, and otherwise communicate with the system controller 106, theoutdoor controller 126, and/or any other device 130 via thecommunication bus 128 and/or any other suitable medium of communication.In some embodiments, the indoor controller 124 may be configured tocommunicate with an indoor personality module 134 that may compriseinformation related to the identification and/or operation of the indoorunit 102. In some embodiments, the indoor controller 124 may beconfigured to receive information related to a speed of the indoor fan110, transmit a control output to an electric heat relay, transmitinformation regarding an indoor fan 110 volumetric flow-rate,communicate with and/or otherwise affect control over an air cleaner136, and communicate with an indoor EEV controller 138. In someembodiments, the indoor controller 124 may be configured to communicatewith an indoor fan controller 142 and/or otherwise affect control overoperation of the indoor fan 110. In some embodiments, the indoorpersonality module 134 may comprise information related to theidentification and/or operation of the indoor unit 102 and/or a positionof the outdoor metering device 120.

In some embodiments, the indoor EEV controller 138 may be configured toreceive information regarding temperatures and/or pressures of therefrigerant in the indoor unit 102. More specifically, the indoor EEVcontroller 138 may be configured to receive information regardingtemperatures and pressures of refrigerant entering, exiting, and/orwithin the indoor heat exchanger 108. Further, the indoor EEV controller138 may be configured to communicate with the indoor metering device 112and/or otherwise affect control over the indoor metering device 112. Theindoor EEV controller 138 may also be configured to communicate with theoutdoor metering device 120 and/or otherwise affect control over theoutdoor metering device 120.

The outdoor controller 126 may be carried by the outdoor unit 104 andmay be configured to receive information inputs, transmit informationoutputs, and otherwise communicate with the system controller 106, theindoor controller 124, and/or any other device via the communication bus128 and/or any other suitable medium of communication. In someembodiments, the outdoor controller 126 may be configured to communicatewith an outdoor personality module 140 that may comprise informationrelated to the identification and/or operation of the outdoor unit 104.In some embodiments, the outdoor controller 126 may be configured toreceive information related to an ambient temperature associated withthe outdoor unit 104, information related to a temperature of theoutdoor heat exchanger 114, and/or information related to refrigeranttemperatures and/or pressures of refrigerant entering, exiting, and/orwithin the outdoor heat exchanger 114 and/or the compressor 116. In someembodiments, the outdoor controller 126 may be configured to transmitinformation related to monitoring, communicating with, and/or otherwiseaffecting control over the outdoor fan 118, a compressor sump heater, asolenoid of the reversing valve 122, a relay associated with adjustingand/or monitoring a refrigerant charge of the HVAC system 100, aposition of the indoor metering device 112, and/or a position of theoutdoor metering device 120. The outdoor controller 126 may further beconfigured to communicate with a compressor drive controller 144 that isconfigured to electrically power and/or control the compressor 116.

The HVAC system 100 is shown configured for operating in a so-calledcooling mode in which heat is absorbed by refrigerant at the indoor heatexchanger 108 and heat is rejected from the refrigerant at the outdoorheat exchanger 114. In some embodiments, the compressor 116 may beoperated to compress refrigerant and pump the relatively hightemperature and high pressure compressed refrigerant from the compressor116 to the outdoor heat exchanger 114 through the reversing valve 122and to the outdoor heat exchanger 114. As the refrigerant is passedthrough the outdoor heat exchanger 114, the outdoor fan 118 may beoperated to move air into contact with the outdoor heat exchanger 114,thereby transferring heat from the refrigerant to the air surroundingthe outdoor heat exchanger 114. The refrigerant may primarily compriseliquid phase refrigerant and the refrigerant may flow from the outdoorheat exchanger 114 to the indoor metering device 112 through and/oraround the outdoor metering device 120 which does not substantiallyimpede flow of the refrigerant in the cooling mode. The indoor meteringdevice 112 may meter passage of the refrigerant through the indoormetering device 112 so that the refrigerant downstream of the indoormetering device 112 is at a lower pressure than the refrigerant upstreamof the indoor metering device 112. The pressure differential across theindoor metering device 112 allows the refrigerant downstream of theindoor metering device 112 to expand and/or at least partially convertto a two-phase (vapor and gas) mixture. The two phase refrigerant mayenter the indoor heat exchanger 108. As the refrigerant is passedthrough the indoor heat exchanger 108, the indoor fan 110 may beoperated to move air into contact with the indoor heat exchanger 108,thereby transferring heat to the refrigerant from the air surroundingthe indoor heat exchanger 108, and causing evaporation of the liquidportion of the two phase mixture. The refrigerant may thereafterre-enter the compressor 116 after passing through the reversing valve122.

To operate the HVAC system 100 in the so-called heating mode, thereversing valve 122 may be controlled to alter the flow path of therefrigerant, the indoor metering device 112 may be disabled and/orbypassed, and the outdoor metering device 120 may be enabled. In theheating mode, refrigerant may flow from the compressor 116 to the indoorheat exchanger 108 through the reversing valve 122, the refrigerant maybe substantially unaffected by the indoor metering device 112, therefrigerant may experience a pressure differential across the outdoormetering device 120, the refrigerant may pass through the outdoor heatexchanger 114, and the refrigerant may reenter the compressor 116 afterpassing through the reversing valve 122. Most generally, operation ofthe HVAC system 100 in the heating mode reverses the roles of the indoorheat exchanger 108 and the outdoor heat exchanger 114 as compared totheir operation in the cooling mode.

Referring now to FIG. 2, a simplified orthogonal front view of outdoorheat exchanger 114 of HVAC system 100 is shown according to anembodiment of the disclosure. While the outdoor heat exchanger 114 isshown in an unbent configuration, the outdoor heat exchanger 114 mayalternatively be bent into a C-shape, U-shape, circular shape, and/orany other suitable configuration to complement the remainder of anoutdoor unit. The outdoor heat exchanger 114 generally comprises anupper end 200 and a lower end 202. The lower end 202 may generally belocated vertically lower than the upper end 200, and in someembodiments, the lower end 202 may be located in close proximity to asupport surface 204 that may generally support the outdoor unit 104 ofFIG. 1.

The outdoor heat exchanger 114 may also comprise a divided header 206and an undivided header 208. The divided header 206 may generallycomprise a tubular structure that comprises an upper volume 210 and alower volume 212. The upper volume 210 and the lower volume 212 maygenerally be separated and prevented from directly communicating fluidbetween each other by a divider 214 disposed within the divided header206 between the upper volume 210 and the lower volume 212. In someembodiments, the divided header 206 may be replaced by two physicallyseparate headers, the upper header comprising the upper volume 210 andthe lower header comprising the lower volume 212. The undivided header208 may generally comprise a substantially similar tubular structure tothat of the divided header 206. However, the undivided header 208 doesnot comprise an internal structure analogous to the divider 214.Accordingly, the undivided header 208 may comprise a substantiallyvertically continuous undivided header volume 216.

The outdoor heat exchanger 114 may also comprise a plurality ofmicrochannel tubes 220 that extend horizontally between the dividedheader 206 and the undivided header 208. The microchannel tubes 220 maygenerally be configured to join the divided header 206 and the undividedheader 208 in fluid communication with each other. The outdoor heatexchanger 114 may also comprise a refrigerant inlet tube 226 insubstantially direct fluid communication with the upper volume 210 ofthe divided header 206. The outdoor heat exchanger 114 may also comprisea refrigerant outlet tube 228 in substantially direct fluidcommunication with the lower volume 212 of the divided header 206.

The microchannel tubes 220 may be configured to join the divided header206 and the undivided header 208 in fluid communication. Themicrochannel tubes 220 that supply refrigerant from the upper volume 210of the divided header 206 to the undivided header 208 may generally bereferred to as supply microchannel tubes 220′, while the microchanneltubes 220 that supply refrigerant from the undivided header 208 to thelower volume 212 of the divided header 206 may be referred to as returnmicrochannel tubes 220″. In some embodiments, the supply microchanneltubes 220′ and the return microchannel tubes 220″ may comprisesubstantially the same length between the divided header 206 and theundivided header 208. It will be appreciated that the outdoor heatexchanger 114 may be described as comprising an upper region 230 thatcomprises the plurality of supply microchannel tubes 220′ and a lowerregion 232 that comprises the plurality of return microchannel tubes220″. It will also be appreciated that the direction of flow ofrefrigerant from the upper volume 210 of the divided header 206 to theundivided header 208 through the supply microchannel tubes 220′ and theflow of refrigerant from the undivided header 208 to the lower volume212 of the divided header 206 through the return microchannel tubes 200″may generally be shown by refrigerant flow arrows 218.

Referring now to FIG. 3, a partial cutaway oblique view of a pluralityof microchannel tubes 220 of the outdoor heat exchanger 114 is shownaccording to an embodiment of the disclosure. In some embodiments, eachmicrochannel tube 220 may comprise a plurality of substantially parallelmicrochannels 222. The microchannels 222 may generally connect thedivided header 206 in fluid communication with the undivided header 208.In some embodiments, the microchannel tubes 220 may comprisemicrochannels 222 that comprise substantially similar diameters. In someembodiments, the microchannel tubes 220 may also comprise asubstantially similar number of microchannels 222. In embodiments wherethe microchannel tubes 220 comprise a substantially similar number ofmicrochannels 222 having substantially similar diameters, it will beappreciated that each microchannel tube 220 may comprise substantiallysimilar microchannel 222 volumes in each microchannel tube 220.Additionally, vertically adjacent microchannel tubes 220 may be joinedto intermediately-disposed fins 224. In some embodiments, theintermediately-disposed fins 224 may be formed from athermally-conductive material and configured to promote heat transferbetween refrigerant flowing through the plurality of microchannel tubes220 and an airflow passing through the outdoor heat exchanger 114 viathe intermediately-disposed fins 224. It will be appreciated that theintermediately-disposed fins 224 are not shown in FIG. 1 for clarity.

Referring now to FIG. 4A, a schematic view of a conventionalmicrochannel heat exchanger 400 illustrating the behavior of liquidrefrigerant in the conventional microchannel heat exchanger 400 undernormal steady state operating conditions is shown. Conventionalmicrochannel heat exchanger 400 may generally comprise a microchannelheat exchanger and be described as similarly configured to outdoor heatexchanger 114 in that conventional microchannel heat exchanger 400comprises an upper end 402, a lower end 404, a divided header 406, andan undivided header 408. The lower end 404 may generally be locatedvertically lower than the upper end 402. The divided header 406 maygenerally comprise a tubular structure that comprises an upper volume410 and a lower volume 412. The upper volume 410 and the lower volume412 may generally be separated and prevented from directly communicatingfluid between each other by a divider 414 disposed within the dividedheader 406 between the upper volume 410 and the lower volume 412. Theundivided header 408 may generally comprise a substantially similartubular structure to that of the divided header 406. However, theundivided header 408 does not comprise an internal structure analogousto the divider 414. Accordingly, the undivided header 408 may comprise asubstantially vertically continuous undivided header volume 416.

Although not shown, the conventional microchannel heat exchanger 400 mayalso comprise a plurality of microchannel tubes, microchannels, and finsthat may be configured substantially similarly to the microchannel tubes220, microchannels 222, and fins 224, respectively, of FIG. 3. Themicrochannel tubes may generally extend horizontally between the dividedheader 406 and the undivided header 408, thereby joining the dividedheader 406 and the undivided header 408 in fluid communication with eachother. The conventional microchannel heat exchanger 400 may alsocomprise a refrigerant inlet tube 418 in substantially direct fluidcommunication with the upper volume 410 of the divided header 406. Theconventional microchannel heat exchanger 400 may also comprise arefrigerant outlet tube 420 in substantially direct fluid communicationwith the lower volume 412 of the divided header 406.

The microchannel tubes may be configured to join the divided header 406and the undivided header 408 in fluid communication. The microchanneltubes that supply refrigerant from the upper volume 410 of the dividedheader 406 to the undivided header 408 may generally be referred to assupply microchannel tubes, while the microchannel tubes that supplyrefrigerant from the undivided header 408 to the lower volume 412 of thedivided header 406 may be referred to as return microchannel tubes. Itwill be appreciated that the conventional microchannel heat exchanger400 may be described as comprising an upper region 422 that comprisesthe plurality of supply microchannel tubes and a lower region 424 thatcomprises the plurality of return microchannel tubes. Generally, theconventional microchannel heat exchanger 400 differs from themicrochannel heat exchanger 114 in that the conventional microchannelheat exchanger 400 may comprise about 50 supply microchannel tubes inthe upper region 422 and about 24 return microchannel tubes in the lowerregion 424. It may alternatively be stated that the conventionalmicrochannel heat exchanger 400 may comprise a conventional microchanneltube configuration that is about ⅔ supply microchannel tubes and about ⅓return microchannel tubes. Accordingly, the conventional microchannelheat exchanger 400 may comprise a ratio of supply microchannel tubes toreturn microchannel tubes that is about 2:1.

Under normal and/or ideal operating conditions in a cooling mode ofoperation, the conventional microchannel heat exchanger 400 may begenerally described as comprising a refrigerant level that is correctlyadjusted, i.e. a level that exists when the closed loop refrigerantsystem is neither substantially overcharged nor substantiallyundercharged. Because, under ideal and/or normal conditions, refrigerantis introduced into the conventional microchannel heat exchanger 400 ashot gas, the hot gas will normally fill the upper volume 410 of thedivided header 406 and travel in parallel paths through the supplymicrochannel tubes of the upper region 422. As the hot gas is cooled byambient outdoor air being forced into contact with the conventionalmicrochannel heat exchanger 400, some of the hot gas may cool andcondense into liquid form before exiting the conventional microchannelheat exchanger 400 through the refrigerant outlet tube 420. Generally, asubstantial amount of such initial condensation and conversion to liquidform may occur in the upper region 422.

When the refrigerant exits the supply microchannel tubes of the upperregion 422, it may be introduced into the undivided header 408 as amixture of condensed liquid and uncondensed hot gas. When the condensedliquid refrigerant reaches the undivided header 408, the refrigerantthat is in liquid form may fall into the bottom of the continuousundivided header volume 416 of the undivided header 408 and becomedistributed into the various return microchannel tubes of the lowerregion 424 of the conventional microchannel heat exchanger 400. Whilerefrigerant passes through the return microchannel tubes of the lowerregion 424, more of the hot gas refrigerant may cool and condense intoliquid form, while previously condensed liquid refrigerant may befurther cooled. Under normal and/or ideal conditions, the lower region424 may comprise a liquid refrigerant volume 426 that is substantiallydistributed across the lower region 424 as shown with the verticallylowest return microchannel tubes completely filling with liquidrefrigerant before relatively higher return microchannel tubes.

Referring now to FIG. 4B, a schematic view of the conventionalmicrochannel heat exchanger 400 illustrating the behavior of liquidrefrigerant in the conventional microchannel heat exchanger 400 underabnormal steady state operating conditions is shown. As opposed tonormal steady state operating conditions, abnormal steady stateoperating conditions may exist when either (1) an HVAC system isovercharged with too much refrigerant and is operating under normaland/or ideal ambient temperature conditions, (2) an HVAC system isproperly charged but is operating under very high ambient temperatureconditions, and/or (3) an HVAC system is both overcharged and isoperating under very high ambient temperature conditions. Under suchdescribed abnormal steady state operating conditions, refrigerantbehavior may be different. For example, in some instances, liquidrefrigerant may back-up into the conventional microchannel heatexchanger 400 through the refrigerant outlet tube 420.

Because liquid refrigerant may back-up into the conventionalmicrochannel heat exchanger 400 through the refrigerant outlet tube 420,some liquid refrigerant may enter the return microchannel tubes of thelower region 424. Liquid refrigerant in the lower region 424 maysubstantially decrease heat transfer and reduce the efficiency of theheat exchanger 400 because liquid refrigerant in the heat exchanger 400may be 80-90% less efficient at transferring heat in the conventionalmicrochannel heat exchanger 400 as compared to gaseous or mixed phaserefrigerant. Additionally, as a result of some steady state operatingconditions causing more condensation of gaseous refrigerant into liquidin the supply microchannel tubes of the upper region 422, a largervolume of liquid refrigerant may enter the undivided header 408, fallinto the bottom of the continuous undivided header volume 416 of theundivided header 408, and become distributed into the various returnmicrochannel tubes of the lower region 424 of the conventionalmicrochannel heat exchanger 400. While refrigerant passes through thereturn microchannel tubes of the lower region 424, more of the hot gasrefrigerant may cool and condense into liquid form, while previouslycondensed liquid refrigerant may be further cooled. Under such abnormalconditions, the lower region 424 may comprise an excess liquidrefrigerant volume 428 that is distributed across the lower region 424substantially as shown and that is substantially greater than the liquidrefrigerant volume 426 shown in FIG. 4A.

Because rows of single phase liquid refrigerant may render portions ofthe conventional microchannel heat exchanger 400 relatively ineffectivefor heat transfer, the excess liquid refrigerant volume 428 may causeabout ⅓ of the heat exchanger 400 to be rendered ineffective for heattransfer purposes under abnormal steady state operating conditions.Thus, the heat exchanger 400 may realize about a ⅓ reduction inefficiency. Additionally, when the excess liquid refrigerant volume 428persists in the lower region 424 of the heat exchanger 400, the excessliquid refrigerant volume 428 may undesirably cause higher subcoolingand/or higher compressor discharge pressures, which in some cases mayultimately cause a compressor, such as compressor 116, to shut off dueto excessively high discharge pressure.

Referring now to FIG. 5A, a schematic view of the outdoor heat exchanger114 illustrating the behavior of liquid refrigerant in the outdoor heatexchanger 114 under normal steady state operating conditions is shownaccording to an embodiment of the disclosure. As stated, outdoor heatexchanger 114 comprises an upper end 200, a lower end 202, a dividedheader 206 that comprises an upper volume 210 and a lower volume 212that is divided by divider 214, an undivided header 208, an upper region230 that comprises a plurality of supply microchannel tubes 220′, and alower region 232 that comprises a plurality of return microchannel tubes220″. It will be appreciated that the supply microchannel tubes 220′ andthe return microchannel tubes 220″ are not shown for clarity. Theoutdoor heat exchanger 114 also comprises a refrigerant inlet tube 226and a refrigerant outlet tube 228.

Under normal and/or ideal operating conditions, the outdoor heatexchanger 114 may generally be described as comprising a refrigerantlevel that is correctly adjusted, i.e. the closed loop refrigerantsystem of HVAC system 100 is neither substantially overcharged norsubstantially undercharged. Because, under ideal and/or normalconditions, refrigerant is introduced into the outdoor heat exchanger114 as hot gas, the hot gas will normally fill the upper volume 210 ofthe divided header 206 and travel in parallel paths through the supplymicrochannel tubes 220′ of the upper region 230. As the hot gas iscooled by ambient outdoor air being forced into contact with the outdoorheat exchanger 114, some of the hot gas may cool and condense intoliquid form, resulting in the upper region 230 comprising somemixed-phase refrigerant. Generally, a substantial amount of such initialcondensation and conversion to liquid form may occur in the upper region230.

When the refrigerant exits the supply microchannel tubes 220′ of theupper region 230, it may be introduced into the undivided header 208 asa mixture of condensed liquid and uncondensed hot gas. When thecondensed liquid refrigerant reaches the undivided header 208, therefrigerant that is in liquid form may fall into the bottom of thecontinuous undivided header volume 216 of the undivided header 208 andbecome distributed into the various return microchannel tubes 220″ ofthe lower region 232 of the outdoor heat exchanger 114.

Generally, as compared to the conventional microchannel heat exchanger400 in FIGS. 4A-4B, the outdoor heat exchanger 114 may comprise areduced number of return microchannel tubes 220″ in the lower region 232relative to the number of supply microchannel tubes 220′ in the upperregion 230 so that the supply microchannel tube to return microchanneltube ratio of the outdoor heat exchanger 114 is substantially greaterthan the supply microchannel tube to return microchannel tube ratio ofthe conventional microchannel heat exchanger 400. In some embodiments,outdoor heat exchanger 114 may comprise only those return microchanneltubes 220″ necessary for subcooling. Accordingly, the plurality ofreturn microchannel tubes 220″ in the lower region 232 of outdoor heatexchanger 114 may be substantially filled by a liquid refrigerant volume500. In some embodiments, reducing the number of return microchanneltubes 220″ in the lower region 232 to only those necessary forsubcooling may allow a substantially larger number of supplymicrochannel tubes 220′ in the upper region 230 as opposed to the heatexchanger 400. Again, the outdoor heat exchanger 114 may comprise ahigher ratio of supply microchannel tubes 220′ in the upper region 230to return microchannel tubes 220″ in the lower region 232 as compared toheat exchanger 400. Most generally, this ratio may be greater than 2:1.In some embodiments, increasing the ratio of supply microchannel tubes220′ to return microchannel tubes 220″ may provide an increase inefficiency over heat exchanger 400. In some embodiments, the increase inefficiency may be at least about 10%, at least about 15%, and/or atleast about 20%.

In some embodiments, outdoor heat exchanger 114 may comprise a ratio ofsupply microchannel tubes 220′ to return microchannel tubes 220″ that isabout 2.5:1, about 3:1, about 4:1, about 5:1, about 6:1, about 7:1,about 8:1, about 9:1, about 10:1, about 11:1, about 12:1, about 13:1,about 14:1, and/or about 15:1. For example, in embodiments where outdoorheat exchanger 114 comprises a ratio of supply microchannel tubes 220′to return microchannel tubes 220″ that is 10:1, outdoor heat exchanger114 may comprise about 60 supply microchannel tubes 220′ and about 6return microchannel tubes 220″. Alternatively, in other embodimentswhere outdoor heat exchanger 114 comprises a ratio of supplymicrochannel tubes 220′ to return microchannel tubes 220″ that is about10:1, outdoor heat exchanger 114 may comprise about 62 supplymicrochannel tubes 220′ and about 6 return microchannel tubes 220″. Itwill be appreciated that an outdoor heat exchanger 114 comprising aratio of supply microchannel tubes 220′ to return microchannel tubes220″ may assume substantially similar volumes through each of themicrochannel tubes 220′, 220″.

Alternatively, in some embodiments, the supply microchannel tubes 220′and the return microchannel tubes 220″ may be characterized and/orconfigured based on their respective microchannel 222 volumes. In suchembodiments, the outdoor heat exchanger 114 may comprise substantiallysimilar microchannel 222 volumes through each of the microchannel tubes220′, 220″. Conversely, in some embodiments, the outdoor heat exchanger114 may comprise supply microchannel tubes 220′ and return microchanneltubes 220″ having dissimilar microchannel 222 volumes. Thus, the outdoorheat exchanger 114 may be characterized by comprising a ratio of themicrochannel 222 volume of the supply microchannel tubes 220′ to themicrochannel 222 volume of return microchannel tubes 220″ that is about2.5:1, about 3:1, about 4:1, about 5:1, about 6:1, about 7:1, about 8:1,about 9:1, about 10:1, about 11:1, about 12:1, about 13:1, about 14:1,and/or about 15:1.

In yet other embodiments, the outdoor heat exchanger 114 may beconfigured based on a frontal area of the upper region 230 to a frontalarea of the lower region 232. The frontal areas of the regions 230, 232may be characterized by the front-facing area that is configured toreceive an airflow supplied by an outdoor fan, such as outdoor fan 118.The frontal area of the upper region 230 may be defined on top by theupper end 200, on bottom by the divider 214, on one side by the dividedheader 206, and on the opposing side by the undivided header 208. Thefrontal area of the lower region 232 may be defined on top by thedivider 214, on bottom by the lower end 202, on one side by the dividedheader 206, and on the opposing side by the undivided header 208.Accordingly, in some embodiments, the outdoor heat exchanger 114 may becharacterized by comprising a ratio of the frontal area of the upperregion 230 to the frontal area of the lower region 232 that is about2.5:1, about 3:1, about 4:1, about 5:1, about 6:1, about 7:1, about 8:1,about 9:1, about 10:1, about 11:1, about 12:1, about 13:1, about 14:1,and/or about 15:1.

In some embodiments, outdoor heat exchanger 114 may be configured suchthat a portion of the liquid refrigerant in the outdoor heat exchanger114 may also occupy at least a portion of the continuous undividedheader volume 216 of the undivided header 208. The portion of the liquidrefrigerant in the outdoor heat exchanger 114 that occupies theundivided header volume 216 may be referred to as a normal liquidrefrigerant header volume 502 that may, in some embodiments under normalsteady state operating conditions, fill the undivided header volume 216to a height that may be substantially about equal to a height of thedivider 214 as measured from the lower end 202. In other embodiments,under normal steady state operating conditions, the normal liquidrefrigerant header volume 502 may comprise a height that is slightlyhigher than the height of the divider 214 as measured from the lower end202.

Referring now to FIG. 5B, a schematic view of an outdoor heat exchanger114 illustrating the behavior of liquid refrigerant in the outdoor heatexchanger 114 under abnormal steady state operating conditions is shownaccording to an embodiment of the disclosure. As stated, abnormal steadystate operating conditions may exist when either (1) the HVAC system 100is overcharged with too much refrigerant and is operating under normaland/or ideal ambient temperature conditions, (2) the HVAC system 100 isproperly charged but is operating under very high ambient temperatureconditions, and/or (3) the HVAC system 100 is both overcharged and isoperating under very high ambient temperature conditions. Under abnormalsteady state operating conditions, the behavior of the refrigerant mayvary. However, unlike the heat exchanger 400 in FIGS. 4A-4B where liquidrefrigerant may back up into the heat exchanger 400 through therefrigerant outlet tube 420 and/or where excess liquid refrigerant mayoccupy a portion of the heat exchanger 400 optimized for gaseous and/ormixed phase refrigerant, outdoor heat exchanger 114 may generally beconfigured to receive excess liquid refrigerant (i.e. amounts of liquidrefrigerant in excess of that which may accommodated by the returnmicrochannel tubes 220″ of the lower region 232) into the undividedheader volume 216 of the undivided header 208.

The outdoor heat exchanger 114 comprises only those return microchanneltubes 220″ necessary for subcooling under normal operating conditions atsteady state such that the liquid refrigerant volume 500 substantiallyfills the return microchannel tubes 220″ of the lower region 232. Whenthe HVAC system 100 is overcharged and/or encounters high ambienttemperature conditions, the amount of liquid refrigerant in the outdoorheat exchanger 114 may increase. In some embodiments, the undividedheader 208 may comprise an undivided header volume 216 configured toreceive a volume of refrigerant that is greater than the total volume ofthe microchannels 222 of the outdoor heat exchanger 114. In someembodiments, the undivided header 208 may comprise an undivided headervolume 216 that is configured to receive about twice as much refrigerantas the microchannels 222 of the outdoor heat exchanger 114. Accordingly,the undivided header 208 of the outdoor heat exchanger 114 may beconfigured to receive the excess liquid refrigerant into the undividedheader volume 216. The portion of the liquid refrigerant in the outdoorheat exchanger 114 that occupies the undivided header volume 216 underabnormal steady state operating condition may thus be referred to as anabnormal liquid refrigerant header volume 504. It will be appreciatedthat the abnormal liquid refrigerant header volume 504 caused byabnormal steady state operating conditions may generally be larger thanthe normal liquid refrigerant header volume 502 incurred under normalsteady state operating conditions. Furthermore, in some embodiments, theabnormal liquid refrigerant header volume 504 may comprise a height thatis substantially higher than the height of the divider 214 as measuredfrom the lower end 202.

Since the excess liquid refrigerant in the outdoor heat exchanger 114caused by abnormal steady state operating conditions may back up intothe undivided header volume 216 of the undivided header 208, portions ofthe lower region optimized for two-phase refrigerant will not bedisplaced in the manner such occurs with heat exchanger 400. As such,even under abnormal steady state operating conditions, the outdoor heatexchanger 114 may provide an increase in efficiency over heat exchanger400. In some embodiments, the increase in efficiency during abnormalconditions may be as high as about 10%, about 15%, and/or about 20%. Insome embodiments, the outdoor heat exchanger 114 may maintain an overallefficiency even when the HVAC system 100 is overcharged or operatingunder excess ambient temperature conditions. Furthermore, because theundivided header 208 may have such a large volume of interior space ascompared to the a sum interior space volumes of a group of and in someembodiments all of the microchannel tubes, the undivided header 208 maybe configured to receive excess liquid refrigerant thereby reducingand/or eliminating a backup of liquid phase refrigerant into the upperregion 230. Accordingly, in spite of the presence of undesirableincreased amounts of liquid phase refrigerant, the outdoor heatexchanger 114 maintains a predetermined amount of utilization of theoutdoor heat exchanger 114 for carrying gaseous and/or mixed phaserefrigerant, thereby maintaining heat exchange efficiency levels andgenerally providing a higher charge tolerance as compared to othermicrochannel heat exchangers, such as conventional microchannel heatexchanger 400.

Referring now to FIG. 6, a flowchart of a method 600 of operating amicrochannel heat exchanger in an HVAC system 100 is shown according toan embodiment of the disclosure. The method 600 may begin at block 602by providing a microchannel heat exchanger in an HVAC system. In someembodiments, the microchannel heat exchanger may comprise outdoor heatexchanger 114. The method may continue at block 604 by introducingrefrigerant into an upper portion of the microchannel heat exchanger.The method 600 may continue at block 606 by condensing a first amount ofliquid phase refrigerant in the upper portion under normal steady stateoperating conditions, wherein the first volume of liquid phaserefrigerant substantially fills the lower portion, and whereinsubstantially none of the refrigerant is condensed into liquid phaserefrigerant in the lower portion. The method 600 may conclude at block608 by condensing a second amount of liquid phase refrigerant in theupper portion under abnormal steady state operating conditions, whereinan additional amount of the second amount of liquid phase refrigerant inexcess of the first amount of liquid phase refrigerant is accommodatedin the undivided header as opposed to further filling the lower portion.

It will be appreciated that in the discussion above, relative volumesand amounts of refrigerant and the spaces occupied by the refrigerantare sometimes discussed as if the refrigerant were not constantlyflowing throughout the closed loop refrigerant system. Such discussionis for illustration purposes only and this disclosure fully contemplatesthat the refrigerant buildup, pooling, presence, and generally behavioris dependent upon dynamic factors such as mass flow rates of therefrigerant, steady operation of HVAC system 100 components such ascompressor 116, and other potentially transient factors.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R_(l), and an upperlimit, R_(u), is disclosed, any number falling within the range isspecifically disclosed. In particular, the following numbers within therange are specifically disclosed: R=R_(l)+k*(R_(u)−R_(l)), wherein k isa variable ranging from 1 percent to 100 percent with a 1 percentincrement, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5percent, . . . , 50 percent, 51 percent, 52 percent, . . . , 95 percent,96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Unlessotherwise stated, the term “about” shall mean plus or minus 10 percentof the subsequent value. Moreover, any numerical range defined by two Rnumbers as defined in the above is also specifically disclosed. Use ofthe term “optionally” with respect to any element of a claim means thatthe element is required, or alternatively, the element is not required,both alternatives being within the scope of the claim. Use of broaderterms such as comprises, includes, and having should be understood toprovide support for narrower terms such as consisting of, consistingessentially of, and comprised substantially of. Accordingly, the scopeof protection is not limited by the description set out above but isdefined by the claims that follow, that scope including all equivalentsof the subject matter of the claims. Each and every claim isincorporated as further disclosure into the specification and the claimsare embodiment(s) of the present invention.

What is claimed is:
 1. A microchannel heat exchanger, comprising: afirst portion connected in fluid communication with an undivided header;and a second portion connected in fluid communication with the undividedheader at a location that is vertically lower on the undivided headerthan the first portion; wherein ratio of the first portion to the secondportion is greater than 2:1.
 2. The microchannel heat exchanger of claim1, wherein the first portion comprises a first set of microchannel tubesand the second portion comprises a second set of microchannel tubes; andwherein the ratio of the first set of microchannel tubes to the secondset of microchannel tubes is greater than 2:1.
 3. The microchannel heatexchanger of claim 1, wherein the first portion comprises a first volumeof microchannels and the second portion comprises a second volume ofmicrochannels; and wherein the ratio of the first volume ofmicrochannels to the second volume of microchannels is greater than 2:1.4. The microchannel heat exchanger of claim 1, wherein the first portioncomprises a first frontal area and the second portion comprises a secondfrontal area; and wherein the ratio of the first frontal area to thesecond frontal area is greater than 2:1.
 5. The microchannel heatexchanger of claim 1, wherein the ratio of the first portion to thesecond portion is at least one of 2.5:1 to 15:1, 3:1 to 15:1, 4:1 to14:1, 5:1 to 13:1, 7:1 to 12:1, 8:1 to 12:1, 9:1 to 11:1, and 9.5:1 to10.5:1.
 6. The microchannel heat exchanger of claim 1, wherein the ratioof the first portion to the second portion is about 10:1.
 7. Themicrochannel heat exchanger of claim 6, wherein the first portioncomprises a first set of microchannel tubes and the second portioncomprises a second set of microchannel tubes; and wherein each of themicrochannel tubes of the first set and each of the microchannel tubesof the second set comprise a substantially similar number ofmicrochannels.
 8. The microchannel heat exchanger of claim 7, whereineach of the microchannel tubes of the first set and each of themicrochannel tubes of the second set comprise substantially similarlengths.
 9. The microchannel heat exchanger of claim 8, wherein each ofthe microchannel tubes of the first set and each of the microchanneltubes of the second set comprise substantially similar microchannelvolumes.
 10. A method of operating a microchannel heat exchanger,comprising: providing a microchannel heat exchanger in an HVAC system,the microchannel heat exchanger comprising an upper portion connected influid communication with an undivided header, and a lower portionconnected in fluid communication with the undivided header at a locationthat is vertically lower on the undivided header than the upper portion,wherein the ratio of the upper portion to the lower portion is greaterthan 2:1; introducing a refrigerant into the upper portion; under normalsteady state operating conditions, condensing a first amount of liquidphase refrigerant in the upper portion, wherein the first volume ofliquid phase refrigerant substantially fills the lower portion, andwherein substantially none of the refrigerant is condensed into liquidphase refrigerant in the lower portion; and under abnormal steady stateoperating conditions, condensing a second amount of liquid phaserefrigerant in the upper portion, wherein an additional amount of thesecond amount of liquid phase refrigerant in excess of the first amountof liquid phase refrigerant is accommodated in the undivided header asopposed to further filling the lower portion.
 11. The method of claim10, wherein the first portion comprises a first set of microchanneltubes and the second portion comprises a second set of microchanneltubes; and wherein the ratio of the first set of microchannel tubes tothe second set of microchannel tubes is greater than 2:1.
 12. The methodof claim 10, wherein the first portion comprises a first volume ofmicrochannels and the second portion comprises a second volume ofmicrochannels; and wherein the ratio of the first volume ofmicrochannels to the second volume of microchannels is greater than 2:1.13. The method of claim 10, wherein the first portion comprises a firstfrontal area and the second portion comprises a second frontal area; andwherein the ratio of the first frontal area to the second frontal areais greater than 2:1.
 14. The method of claim 10, wherein the ratio ofthe first portion to the second portion is at least one of 2.5:1 to15:1, 3:1 to 15:1, 4:1 to 14:1, 5:1 to 13:1, 7:1 to 12:1, 8:1 to 12:1,9:1 to 11:1, and 9.5:1 to 10.5:1.
 15. The method of claim 10, whereinthe ratio of the first portion to the second portion is about 10:1. 16.The method of claim 15, wherein the first portion comprises a first setof microchannel tubes and the second portion comprises a second set ofmicrochannel tubes; and wherein each of the microchannel tubes of thefirst set and each of the microchannel tubes of the second set comprisea substantially similar number of microchannels.
 17. The method of claim16, wherein each of the microchannel tubes of the first set and each ofthe microchannel tubes of the second set comprise substantially similarlengths.
 18. The method of claim 17, wherein each of the microchanneltubes of the first set and each of the microchannel tubes of the secondset comprise substantially similar microchannel volumes.
 19. The methodof claim 18, wherein abnormal operating conditions comprise at least oneof (1) the HVAC system is overcharged with too much refrigerant and isoperating under normal ambient temperature conditions, (2) the HVACsystem is properly charged but is operating under very high ambienttemperature conditions, and (3) the HVAC system is both overcharged andis operating under very high ambient temperature conditions.